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Paralleling AC Generators
Most electrical power grids and distribution systems have more than one AC generator operating at one time. Normally, two or more generators are operated in parallel in order to increase the available power. Three conditions must be met prior to paralleling (or synchronizing) A C generators. 1. Their terminal voltages must be equal. If the voltages of the two AC generators are not equal, one of the AC generators could be picked up as a reactive load to the other AC generator. This causes high currents to be exchanged between the two machines, possibly causing generator or distribution system damage. 2. Their frequencies must be equal. A mismatch in frequencies of the two AC generators will cause the generator with the lower frequency to be picked up as a load on the other generator (a condition referred to as "motoring"). This can cause an overload in the generators and the distribution system.
3. Their output voltages must be in phase. A mismatch in the phases will cause l arge opposing voltages to be developed. The worst case mismatch would be 180° ou t of phase, resulting in an opposing voltage between the two generators of twice the output voltage. This high voltage can cause damage to the generators and distribution system due to high currents.
During paralleling operations, voltages of the two generators that are to be paralleled ar e indicated through the use of voltmeters. Frequency matching is accomplished through the use of output frequency meters. Phase matching is accomplished through the use of a synchroscope, a device that senses the two frequencies and gives an indication of phase differences a nd a relative comparison of frequency differences.
CURRENT CIRCULATING IN PARALLEL AC GENERATORS
When two or more generator sets are operated in parallel, a current may circulate between the generators. This current will exist when the internal voltage generated by each generator is slightly different, but the terminal or bus voltage is the same. In the most elementary form, current will flow out of the line leads of on generator, through the paralleling bus & into the second generator. It does not flow into the load. This current, called "Circulating Current", is in addition to the normal line current supplied to the connected load. When more than two generators are in parallel, current could flow out of any generator & into one or more of the other generators. Circulating currents can take many paths into & out of the several generators. We are concerned with these "Wattless amperes" only when they interfere with normal generator set operation or when the normal on-line KVA capacity of the generators must be reduced because of excessive line currents. With no-load (zero kilowatts) on a generator in a parallel system.
Since circulating currents pass through the generator coils, these currents heat the coils the same way as does the load current. Further, since circulating currents are superimposed on the load current passing through the circuit breaker, circulating currents can cause a breaker to trip as the breaker could "see" an actual ampere overload. More complex control systems include "reverse current" relays which sense counter flow currents. Currents in excess of the relay getting will actuate the circuit breaker trip mechanism. Observed line current (as indicated by panel ammeters) in a parallel generator set system is a summation of two or three currents: Load current-that current which is supplied to the load. It may be in phase with the voltage (unity power factor) or somewhat out of phase with the voltage (power factor less than unity). Harmonic current-usually third harmonic current which flows through the entire system when "Y" connected paralleled generators have their neutral leads connected, either directly or through an earth or ground connection. Circulating current-that current which flows between generators
Parallel Operation of two Generators
When two synchronous generators are connected in parallel, they have an inherent tendency to remain in step, on account of the changes produced in their armature currentsby a divergence of phase. Consider identical machines 1 and 2, Fig. 35 in parallel and working on to the same load. With respect to the load, their e.m.fs are normally in phase: with respect to the local circuit formed by the two armature windings, however, their e.m.fs are in phase-opposition. Suppose there to be no external load. If machine 1 for some reason accelerates, its e.m.f. will draw ahead of that of machine 2. The resulting phase difference 2_ causes e.m.fs to lose phase-opposition in the local circuit so that there is in effect a local e.m.f Es which will circulate a current Is in the local circuit of the two armatures. The current Is flows in the synchronous impedance of the two machines together, so that it lags by _ = arc tan(xs/r) _ 90 on Es on account of the preponderance of reactance inZsIs therefore flows out of machine 1 nearly in phase with the e.m.f., and enters 2 in opposition to the e.m.f. Consequently machine 1 produces a power Ps _ E1Is as a generator, and supplies it (I2R losses excepted) to 2 as a synchronous motor. The synchronizing power Ps tends to retard the faster machine 1 and accelerate the slower, 2, pulling the two back into step. Within the limits of maximum power, therefore, it is not possible to destroy the synchronous running of two synchronous generators in parallel, for a divergence of their angular positions results in the production of synchronizing power, which loads the forward machine and accelerates the backward machine to return the two to synchronous running. The development of synchronizing power depends on the fact that the armature impedance is preponderating reactive. If it were not, the machines could not operate stably in parallel: for the circulating current Is would be almost in phase- quandrature
with the generated e.m.f.¶s, and would not contribute any power to slow the faster or speed up the slower machine. When both machines are equally loaded pn to an external circuit, the synchronizing power is developed in the same way as on no load, the effect being to reduce the load of the slower machine at the same time as that of the faster machine is increased. The conditions are shown in Fig., where I1, I2 are the equal load currents of the two machines before the occurrence of phase displacement, and I are the currents as changed by the circulation of the synchronizing current Is. The argument above has been applied to identical machines. Actually, it is not essential for them to be identical, nor to have equal excitations nor power supplies.
In general, the machines will have different synchronous impedance Zs1, Zs2; different e.m.f.¶s E1 and E2 and different speed regulations. The governors of prime movers are usually arranged so that a reduction of the speed of the prime mover is necessary for the increase of the power developed. Unless the governor speed/load characteristics are identical the machines can never share the total load in accordance with their ratings. The governor characteristics take the form shown in Fig. 36. If the two are not the same, the load will be shared in accordance with the relative load values at the running speed, for synchronous machines must necessarily run at identical speeds.
Parallel System Benefits
y y y y y y
Switch loads from one generator to the other without shutting down complete system Peace of mind - always one generator ready to supply the system in reserve One smaller generator can efficiently cover lower power demands Peak Performance - cover demanding peak periods by running generators together. Cover unexpectedly larger power demands Flexible - when not enough room for a single large generator is available
The Advantages of the Parallel Running of Two AC Generators in Operation
When you run a generator, certain thoughts must be taken into account. First, the power requirements of the load has to be considered, along with the overall output of the generator. Second, the size of the generator must be taken into account, to supply the required electricity. By running two smaller generators in parallel, you gain advantages over running just a single large generator.
Output Reduction Advantage
By running two generators in parallel, you reduce the output each one is required to put forth. For example, suppose you calculated out the load to be 1,000 watts. Running just one generator, it will have to produce 1,000 watts of power. By running two generators in parallel, the wattage output is divided by two, so each output is 500 watts. A 500-watt generator is lighter, smaller and more transportable than a 1,000-watt unit. If the generators are mounted on a wind turbine, less bracing is needed on the tower.
System Failure Advantage
If you were using just one generator, the entire charging system would have to be shut down if it failed. With two generators, if one fails the system can keep running until the faulty one is repaired or replaced. The power output is reduced by half, so conservation steps have to be enacted while the system is under repair. Even so, the entire system can keep running.
Back Up Generator Advantage
Two generators can run during peak power times, one can be cut off during low power times. For marine systems, this is a critical advantage. For example when air conditioning is needed, both can be run. When temperatures cool down, only one generator is needed, and the other one shut off. This leads to fuel savings while the ship or boat is in mid-voyage, with no source of fuel nearby.
Most electrical power grids and distribution systems have more than one AC generator operating at one time. Normally, two or more generators are operated in parallel in order to increase the available power. Three conditions must be met prior to paralleling (or synchronizing) A C generators. 1. Their terminal voltages must be equal. If the voltages of the two AC generators are not equal, one of the AC generators could be picked up as a reactive load to the other AC generator. This causes high currents to be exchanged between the two machines, possibly causing generator or distribution system damage. 2. Their frequencies must be equal. A mismatch in frequencies of the two AC generators will cause the generator with the lower frequency to be picked up as a load on the other generator (a condition referred to as "motoring"). This can cause an overload in the generators and the distribution system.
3. Their output voltages must be in phase. A mismatch in the phases will cause l arge opposing voltages to be developed. The worst case mismatch would be 180° ou t of phase, resulting in an opposing voltage between the two generators of twice the output voltage. This high voltage can cause damage to the generators and distribution system due to high currents.
During paralleling operations, voltages of the two generators that are to be paralleled ar e indicated through the use of voltmeters. Frequency matching is accomplished through the use of output frequency meters. Phase matching is accomplished through the use of a synchroscope, a device that senses the two frequencies and gives an indication of phase differences a nd a relative comparison of frequency differences.
CURRENT CIRCULATING IN PARALLEL AC GENERATORS
When two or more generator sets are operated in parallel, a current may circulate between the generators. This current will exist when the internal voltage generated by each generator is slightly different, but the terminal or bus voltage is the same. In the most elementary form, current will flow out of the line leads of on generator, through the paralleling bus & into the second generator. It does not flow into the load. This current, called "Circulating Current", is in addition to the normal line current supplied to the connected load. When more than two generators are in parallel, current could flow out of any generator & into one or more of the other generators. Circulating currents can take many paths into & out of the several generators. We are concerned with these "Wattless amperes" only when they interfere with normal generator set operation or when the normal on-line KVA capacity of the generators must be reduced because of excessive line currents. With no-load (zero kilowatts) on a generator in a parallel system.
Since circulating currents pass through the generator coils, these currents heat the coils the same way as does the load current. Further, since circulating currents are superimposed on the load current passing through the circuit breaker, circulating currents can cause a breaker to trip as the breaker could "see" an actual ampere overload. More complex control systems include "reverse current" relays which sense counter flow currents. Currents in excess of the relay getting will actuate the circuit breaker trip mechanism. Observed line current (as indicated by panel ammeters) in a parallel generator set system is a summation of two or three currents: Load current-that current which is supplied to the load. It may be in phase with the voltage (unity power factor) or somewhat out of phase with the voltage (power factor less than unity). Harmonic current-usually third harmonic current which flows through the entire system when "Y" connected paralleled generators have their neutral leads connected, either directly or through an earth or ground connection. Circulating current-that current which flows between generators
Parallel Operation of two Generators
When two synchronous generators are connected in parallel, they have an inherent tendency to remain in step, on account of the changes produced in their armature currentsby a divergence of phase. Consider identical machines 1 and 2, Fig. 35 in parallel and working on to the same load. With respect to the load, their e.m.fs are normally in phase: with respect to the local circuit formed by the two armature windings, however, their e.m.fs are in phase-opposition. Suppose there to be no external load. If machine 1 for some reason accelerates, its e.m.f. will draw ahead of that of machine 2. The resulting phase difference 2_ causes e.m.fs to lose phase-opposition in the local circuit so that there is in effect a local e.m.f Es which will circulate a current Is in the local circuit of the two armatures. The current Is flows in the synchronous impedance of the two machines together, so that it lags by _ = arc tan(xs/r) _ 90 on Es on account of the preponderance of reactance inZsIs therefore flows out of machine 1 nearly in phase with the e.m.f., and enters 2 in opposition to the e.m.f. Consequently machine 1 produces a power Ps _ E1Is as a generator, and supplies it (I2R losses excepted) to 2 as a synchronous motor. The synchronizing power Ps tends to retard the faster machine 1 and accelerate the slower, 2, pulling the two back into step. Within the limits of maximum power, therefore, it is not possible to destroy the synchronous running of two synchronous generators in parallel, for a divergence of their angular positions results in the production of synchronizing power, which loads the forward machine and accelerates the backward machine to return the two to synchronous running. The development of synchronizing power depends on the fact that the armature impedance is preponderating reactive. If it were not, the machines could not operate stably in parallel: for the circulating current Is would be almost in phase- quandrature
with the generated e.m.f.¶s, and would not contribute any power to slow the faster or speed up the slower machine. When both machines are equally loaded pn to an external circuit, the synchronizing power is developed in the same way as on no load, the effect being to reduce the load of the slower machine at the same time as that of the faster machine is increased. The conditions are shown in Fig., where I1, I2 are the equal load currents of the two machines before the occurrence of phase displacement, and I are the currents as changed by the circulation of the synchronizing current Is. The argument above has been applied to identical machines. Actually, it is not essential for them to be identical, nor to have equal excitations nor power supplies.
In general, the machines will have different synchronous impedance Zs1, Zs2; different e.m.f.¶s E1 and E2 and different speed regulations. The governors of prime movers are usually arranged so that a reduction of the speed of the prime mover is necessary for the increase of the power developed. Unless the governor speed/load characteristics are identical the machines can never share the total load in accordance with their ratings. The governor characteristics take the form shown in Fig. 36. If the two are not the same, the load will be shared in accordance with the relative load values at the running speed, for synchronous machines must necessarily run at identical speeds.
Parallel System Benefits
y y y y y y
Switch loads from one generator to the other without shutting down complete system Peace of mind - always one generator ready to supply the system in reserve One smaller generator can efficiently cover lower power demands Peak Performance - cover demanding peak periods by running generators together. Cover unexpectedly larger power demands Flexible - when not enough room for a single large generator is available
The Advantages of the Parallel Running of Two AC Generators in Operation
When you run a generator, certain thoughts must be taken into account. First, the power requirements of the load has to be considered, along with the overall output of the generator. Second, the size of the generator must be taken into account, to supply the required electricity. By running two smaller generators in parallel, you gain advantages over running just a single large generator.
Output Reduction Advantage
By running two generators in parallel, you reduce the output each one is required to put forth. For example, suppose you calculated out the load to be 1,000 watts. Running just one generator, it will have to produce 1,000 watts of power. By running two generators in parallel, the wattage output is divided by two, so each output is 500 watts. A 500-watt generator is lighter, smaller and more transportable than a 1,000-watt unit. If the generators are mounted on a wind turbine, less bracing is needed on the tower.
System Failure Advantage
If you were using just one generator, the entire charging system would have to be shut down if it failed. With two generators, if one fails the system can keep running until the faulty one is repaired or replaced. The power output is reduced by half, so conservation steps have to be enacted while the system is under repair. Even so, the entire system can keep running.
Back Up Generator Advantage
Two generators can run during peak power times, one can be cut off during low power times. For marine systems, this is a critical advantage. For example when air conditioning is needed, both can be run. When temperatures cool down, only one generator is needed, and the other one shut off. This leads to fuel savings while the ship or boat is in mid-voyage, with no source of fuel nearby.
